This invention relates to fiber optic acoustic sensors and more particularly to a rigid-mandrel based acceleration insensitive extended fiber optic interferometric hydrophone that is adaptable for inclusion in a serially connected array thereof that can be wrapped around a drum of convenient size.
A hydrophone is essentially an acoustic pressure sensor that is designed for underwater use, e.g., for detecting acoustic wave signals in the ocean. Hydrophone sensors and systems are generally described in the book "How To Build and Use Low-Cost Hydrophones" by Frank Watlington, Tab Books, Blue Ridge Summit, Pa. 17214, Copyright 1979. Fiber optic hydrophones are described in U.S. Pat. No. 4,162,397, Fiber Optic Acoustic Sensor by J. A. Bucaro, et al; 4,525,818, Stable Fiber-Optic Hydrophone by P. G. Cielo, et al; and 4,570,248, Interferometric Hydrophone Reference Leg Low Frequency Compensation by G. L. Assard; and the article Optical Fiber Sensor Technology by T. G. Giallovenzi, et al, IEEE Journal of Quantum Electronics, Vol. QE-18, No. 4, April 1982, pp 626-665.
A hydrophone system generally comprises a plurality of hydrophone sensors that are towed behind a ship. Although descriptions of fiber optic hydrophone sensors are beginning to appear in the literature, the conventional method for the detection of sound in water employs piezoelectric sensors attached to electrical equipment that is also located underwater. This means that electrical power must be supplied to the underwater equipment. This is undesirable since seals in the underwater sensor may leak and cause equipment malfunction and it adds to the weight, bulk, complexity, and cost of a system using such sensors. Also, piezoelectric sensors have limited sensitivity, are prone to pick up extraneous electromagnetic signals, and can be acceleration sensitive. A double tube transducer employing an acceleration cancellation technique is illustrated at page 41 of the Watlington book, Supra.
In a fiber optic hydrophone system, an optical source and photodetector are located on a towing vessel and connected through one or a plurality of optical fibers to a trailing network of fiber hydrophones. Each hydrophone sensor for use in the towed array generally comprises a Mach-Zehnder interferometer, for example, that is formed by two lengths of fiber (arms) that are joined together at opposite ends thereof by couplers that are connected to either input or output bus fibers. One length of fiber (the sense arm) is made sensitive to an incident acoustic pressure wave, while the other length of fiber (the reference arm) is intentionally made substantially insensitive to the incident acoustic pressure by isolating it from the latter. If the reference arm is not well isolated from the acoustic signal and responds to it with the same phase as the sense arm, then the resulting hydrophone can exhibit poor sensitivity. This results from the fact that an optical interferometer senses the difference in optical path between the arms. Techniques that have been previously employed for enhancing the sensitivity of a fiber hydrophone include applying a plastic jacketing material such as nylon over a long length of sense fiber that is wound on the circumference of a rigid-extended (i.e., elongated) mandrel, winding the sense fiber around a thin walled cylindrical shell that is subsequently filled with a pliable epoxy material, backing the sense fiber with an air filled cavity, and fabricating cylindrical sense and reference mandrels out of a compliant plastic material such as nylon or teflon.
One fiber optic hydrophone for use in a towed array comprised sense and reference mandrels that were elongated cylinders having raised circular shoulders or flanges at the centers thereof and that were made out of nylon and ceramic, respectively. The outer dieter of the circular flange on the reference mandrels was dimensioned to make a sliding fit in the interior of the sense mandrel. Separate lengths of single mode fiber that had a prescribed optical pathlength difference (OPD) between them were wound on the circumferences of these mandrels and connected to input and output couplers. The reference mandrel was then inserted into and attached to the sense mandrel at the central shoulder points by screws to provide an acceleration insensitive pulling point. After the optical couplers were located inside the reference mandrel, the interior of the reference mandrel and the space between it and the sense mandrel was filled with a resilient epoxy material. This hydrophone sensor was temperature sensitive, failed under pressure cycling, and had poor isolation of the reference arm from the incident acoustic signal and a resultant low sensitivity. This poor sensitivity is believed to be caused by the signal in the reference arm canceling the signal in the sense arm. The hydrophone failures are believed to have been caused by making the mandrels out of different materials that have different properties and which react differently to thermal and pressure changes. Additionally, an epoxy material is not sufficient to dampen the incident acoustic wave and isolate it from the reference winding.
An object of this invention is the provision of an improved acoustic sensor.